I have a lack of general conversation topics this morning, so I'll recall a thought I had on the way to the cafe.
On the way to the cafe I saw I sign that read "50% off Moving Supplies" and then sarcastically thought "Which means they're only half-decent now". This got me to wonder, what exactly does that phrase mean. Obviously the intention of the phrase is to say that the price, while still being more than what we would expect, is much closer to what we expect. There are two ways this particular phrasing could be interpreted.
One, and the most straight-forward, is that the price of the item is half of what is a decent price would be, yet that actually means the price is lower than what would be a decent price, so from a consumer stand-point there would be no justification of complaining.
A second interpretation, in a very round-about way, is to say that half of the (new) price of the item is what would actually be a decent price, so it is still twice what would be a decent price. This conveys the actual intention of the original phrase, yet isn't immediately clear from the wording and involves the assumption of additional wording to create the association of words for this meaning.
On the other hand we quite often use the inverse connotation of the phrase, such as "The food was half-decent", to actually mean that it surpassed decent by either half or twice a decent amount, depending on your view. Once again in this case we mean the second interpretation, but in a positive way. So while the phrase's most direct interpretation is one, we seem to always mean the second.